# Is this NPN transistor AND logic gate practical?

Most places on the internet that I see give a schematic for an AND gate that is something like this:

simulate this circuit – Schematic created using CircuitLab

The only thing about this is that when B is turned on but not A, which should logically output false, it seems to be somewhere between it (this can easily be seen with an LED at the output: it isn't as bright as it is if both A and B are on, logically satisfying AND, but it still lights up).

I was messing around and came up with this idea for an AND logic gate which works a bit more correct and I have been using it recently:

simulate this circuit

Essentially what I came up with when building that is Input A is put through a NOT gate which is put into another NOT gate, but the Collector on the second NOT gate is only powered by Input B. Therefore, if Input B is off, no output to begin with, but if its on but A isn't, then the first NOT gate powers the base to the second, making B ground.

I like building this design better, and it works so far for me, but I was wondering:

How practical is this design (e.g. why haven't I seen it before; are there any flaws)?

It does work as an AND gate, but there is no gain for input B -- so you can't use its output to drive another gate's Input B.

For a logic family to work well, you need to have the output signal go high and low, and also be able to drive a number (fan out) of similar gates.

You could make a gate with input A and the V+ input of Q1 -- it would generate OUT = A & not(B).

That gate would have gain.

• I see. I haven't used it to where the output would be put through enough resistors to make it useless, but like you said, the output couldn't be fed to another AND gate easily. Since the project that I am using this logic gate for is literally nothing but logic gates, I can just be careful when connecting outputs (breadboards help with knowing if two circuits work together before I solder). Thanks! – Cello Coder Nov 25 '15 at 7:15

What you are talking about is a form of RTL (Resistor-Transistor Logic). Google for "RTL circuits" to see examples of how to implement logic functions in this way.